Overloaded. Applies an accumulator function over a sequence. The specified seed value is used as the initial accumulator value, and the specified function is used to select the result value.(Defined by Enumerable.)

Overloaded. Groups the elements of a sequence according to a key selector function. The keys are compared by using a comparer and each group's elements are projected by using a specified function.(Defined by Enumerable.)

Overloaded. Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. The keys are compared by using a specified comparer.(Defined by Enumerable.)

Overloaded. Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. The elements of each group are projected by using a specified function.(Defined by Enumerable.)

Overloaded. Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. Key values are compared by using a specified comparer, and the elements of each group are projected by using a specified function.(Defined by Enumerable.)

Overloaded. Projects each element of a sequence to an IEnumerable<T>, and flattens the resulting sequences into one sequence. The index of each source element is used in the projected form of that element.(Defined by Enumerable.)

Overloaded. Projects each element of a sequence to an IEnumerable<T>, flattens the resulting sequences into one sequence, and invokes a result selector function on each element therein.(Defined by Enumerable.)

Overloaded. Projects each element of a sequence to an IEnumerable<T>, flattens the resulting sequences into one sequence, and invokes a result selector function on each element therein. The index of each source element is used in the intermediate projected form of that element.(Defined by Enumerable.)

Overloaded. Returns the only element of a sequence, or a default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence.(Defined by Enumerable.)

Overloaded. Returns the only element of a sequence that satisfies a specified condition or a default value if no such element exists; this method throws an exception if more than one element satisfies the condition.(Defined by Enumerable.)

Overloaded. Bypasses elements in a sequence as long as a specified condition is true and then returns the remaining elements. The element's index is used in the logic of the predicate function.(Defined by Enumerable.)

Each pair must have a unique key. Implementations can vary in whether they allow key to be null. The value can be null and does not have to be unique. The IDictionary<TKey, TValue> interface allows the contained keys and values to be enumerated, but it does not imply any particular sort order.

The foreach statement of the C# language (For Each in Visual Basic, for each in C++) returns an object of the type of the elements in the collection. Since each element of the IDictionary<TKey, TValue> is a key/value pair, the element type is not the type of the key or the type of the value. Instead, the element type is KeyValuePair<TKey, TValue>. For example:

Notes to Implementers:

The following code example creates an empty Dictionary<TKey, TValue> of strings, with integer keys, and accesses it through the IDictionary<TKey, TValue> interface.

The code example uses the Add method to add some elements. The example demonstrates that the Add method throws ArgumentException when attempting to add a duplicate key.

The example uses the Item property (the indexer in C#) to retrieve values, demonstrating that a KeyNotFoundException is thrown when a requested key is not present, and showing that the value associated with a key can be replaced.

The example shows how to use the TryGetValue method as a more efficient way to retrieve values if a program often must try key values that are not in the dictionary, and how to use the ContainsKey method to test whether a key exists prior to calling the Add method.

Finally, the example shows how to enumerate the keys and values in the dictionary, and how to enumerate the values alone using the Values property.